Signaling system



Patented Mar. 4, 1930 UNITED, STATES' PATENT OFFICE HARRY NYQUIST, 0FMILLBIURN, AND KENNETH W. PFLEGER, 0F ARLINGTON, NEW JERSEY, ASSIGNORSTO AMERICAN TELEPHONE AND TELEGRA'PE COMPANY, A

CORPORATION OF NEW YORK Application filed Jinic 11,

It is an object of our invention to provide for effectively transmittinga composite current comprising component currents of frequencies in aband from zero up to a considerable value. Another object of ourinvention is eectively to transmit the lower end part of such a band offrequencies by cutting, it ofi and stepping it up in frequency above theupper end of the band for actual'v frequency somewhat above saidlimiting value. Still another object of our lnvention is to-separate thecomponents of a signal into separate channels according to theirfrequency above and below acertain critical intermediate frequency, andthen to recombine them with proper attenuation equali- `zation vand.phase correction so that as recombined `the resultant signal wave willcorrespond accurately with the initial signal wave. All these` objectsand other objects of our invention will be made apparent in thefollowing specification and claims taken with the accompanyin,grdrawings, in which we have disclosed an example of the practice of ourinvention. yIt will be understood that this disclosure is specific tothis example, and that the scope of' the invention is intended to beindicated in the appended clams. V In the drawings, Figure l is adiagram of apparatus at the transmitting end of a signaling system;.l `if. i2 is a diagram of appa- 4ratus at thereceiving end; and Figs. 3 to 6are coordinate diagrams of amplitude and phase shift as functions offrequency.

' To illustrate our invention, it will be.

shown how an increase in speed of telegraph transmission may be obtainedover a telephone line whose band of transmitted frequencies is limited.This inventionlis appli- SIGNLING SYSTEM i 1927. seran m. 198,285.

ing a total transmitted band of l800 cycles in width. If the speed isdoubled, then a line would be required which transmits twice that band,or 1,600 cycles. If the speed is again doubled, a line would be requiredwhich transmits twice 1,600. or 3,200 cycles. This assumes, of course,that sending and receiving mechanisms can be devised to operate at thesespeeds. The latter speed cannot be handled by medium-heavy loadedcircuits since they cut of at about 2,800 cycles. Neither is it possibleto transmit the envelope of theV carrier wave as a series of D. C.pulses ranging in frequency components from zero to v 1,600 cyclesbecause the line and apparatus do not transmit the very low frequencies.But it is possible to obtain this highest speed with such a line whenthe very low-frequency components are separated from the rest of thesignal and transmitted by carrier waves. Figs. 1 and 2 show anapplication of this method to the transmission of a signal includingfrequencies from zero to 1.500 cycles. This range is indicated in Fig.6. It is assumed that the cable does not transmit frequencies much below200 cycles. ,The filters located at A and B in Fig. 1 divide theoriginal signal into two paths,-one transmitting .from zero to about 200cycles and the other from about 200 to 1.500 cycles. The currents ofthe/path A are too low in frequency to be transmitted over the cable andconsequently are used to modulate an 1,800-,cycle carrier wave, theproducts of modulation being about 1,600 to 2.000 cycles. which areeasily transmitted by the cable. The currents from the two paths a-recombinedand transmitted over the cable, the frequencies ranging from 200to 2.000 cycles.

A carrier Wave of 1,800; cycles is used inlthan 200 cycles, while inpath stead of 1,700 cycles (which would give 'prod- .ucts of modulationfrom 1,500 to 1,900 cycles) in order to allow for the fact that thefilters cannot be made to cut off sharply and to avoid overlapping ofthe lo'wcr side band with the frequencies below 1,500 cycles. This 'ismade apparent in Fig. 6. 'At the receiving end ofthe line the ilter's inpaths A and B" in Fig. 2 separate the products of modulation from theother frequencies. The former pass through-a detector and reappear atthe output as currents of zero to 200 cycles. Transmission equalizersare shown in order ,to adjust the amplitudes of -the currents ofeackpathto the proper values so that on combinlng them the original signal willbe-exactly reproduced, provided the currents are all in the correctphase relation. The details of construction of transmission equalizers,phase equalizers, and filters are well lmown and therefore they will notbe given here.

The correct phase relation for distortionless transmission is'obtainedJoy means of.

phase equalizers which may be separate from the transmission equalizingnetworks. Such phase equalizers may consist of lattice type all-passnetworks or their equivalent. Due to the fact that the filters used inFig. 1 do not cut oil'I sharply at 200 cycles it will befound that inthe path A of Fig. 2, currents will exist of frequencies sli htlygreater there will be some frequencies less than 200. cycles. It isnecessary that the phases of currents-in this overlapping portion of thefrequency band beso adjusted that 'where paths A and B join, thecurrents will not cancel due to being in phase opposition. In choosingthe transmission and phase equalizers of Fig. Y 2, it is necessary toknow what-phase displacement and amplitude each current -in path A hasat the output of the detector and likewise in path B at the output ofthe low-pass filter. This" information may be obtained either bycomputation or by measurement after the circuit has been set up. Inorder that the combined currents should give thev exact reproduction'ofthe original signal it is first necessary that currents of allfrequencies, 04,500 cycles be transmitted e ually. ,'lherefore, thetransmission equalizers Nos. 1 and 2 should be arran ed to -correct theamplitudes according to ig. 3 so that when currents of the two paths arecombined in phase the resulting amplitude-frequency curve will beastraight line, equal to the sum of the two curves in F ig. 3.

Thephase of currents in paths A and B measured before correction may beplotted as two separate `curves shown in Fig. 4 which may o' mostlikely', may not be straight lines. Unies e. curves-coincide over thefrequency `,tmmnuori@te'.jiboth paths or are parallel and u' lmultipleet360o apart then the two multiple of 360 apart throughout this range.

The system disclosed. will be dependent for success on the design ofthese networks. Perhaps the simplest networks to construct areladder-type filters. One or more sections of simple low pass filter,with series inductance and shunt'capacity, may be used for the phaseequalizer of path'A in Fig. 2f The phase shift per section in thenon-attenuation range, is given by the formula:

Sm f. where fc=cutoff frequency of the filter. The phase shift is' zeroat zero frequency and increases faster ,and faster with increase' infrequency up to the cut-0H point. By locating, f., above 250 cycles thedesired phase shift may be obtained (concave upwardlwhich,

added to the curve for path A of Fig. 4,-

gives the curve for path AUof Fig. 5. The number of sections and thelocation of fc for each' section is an adjustment to be determined bythedesigner cfa particular installation.y or example, by raisin can be madeless concave, whilegby increasing the number of sections the curve canbe made steeper, and the total phase shift increased.

In calculating the total phaseI shift caused Joya` a number of sectionsof filter it is, of course,

ln'ecessary, for accurate results, to take account of the impedancesconnected to the filter.

The phase Aequalizer' of-pa'th B in- Fig. '2 may consist of oneor moresections of simple c, the curve 'high pass filter, with series capacityand shunt'inductance. The phase shift per section in the'non-attenuating range is given by the formula: f

The phase shift is Hzero at ,infinite fre uency and increases negativelywith decrease 1n frequency down to the cut-oil' point. B locating fcbelow 150 cycles, the desire phase shiftvm'ay be obtained (concavedownward) which, when added to the curve for path B of Fig. 4 gives thecurve for path B of the location of the respective cut-off Afrequenciesis determined by the designer for each installation just as in the caseofthe low pass filters.l It is also possible to accomplish the desiredresult by the use of latticetype all-pass networks or their equivalentFig. 5. The exact number of sections and I which-have the advantage ofpossessin a constant characteristic impedance lat all requencies, whichsimplifies the calculations when they are terminated inmatchedjmpedances. .In this case it is only necessary to consider theequation of each section which givesthe phase shift: l ,v

.'B with theirfo above 200 cycles. By taking the `right number ofsections and adjusting the constant I) of` each network, the slopes ofthe resulting phase curves for each path may be varied until theycometan ent or meetthe l specifications outlinedabove or proper joiningofthe paths. In'order to obtain a180 phase shift whenever necessary theconnections to. one of thep'aths maybe reversed.

For accurate work it is necessary, of course,

that the phase and transmission equalizers be designed to allow for thefact that in practical'construction the former will have some effect onthe attenuation and thelatter some v eft'ecton the phase.

paths A and B will then have the desired amplitude characteristic butprobably a somewhat irregular delay curve. The phase equalize'r directlyon the input side of the receiver in Fig. 2, consisting of lattice-typenetworks, may e employed 1n the customary manner, to give a flat delaycurve-to currents entering thereceiving device. When'all frequencies aretransmitted' with substantially equal lamplitudes and delay,'the signalsvare practically ree from' distortion. e r o Weclaim: p e ,l l 1. Themethod ofy transmitting a signal,

A,which consists in separating its components into respective channelsapproximately above and below a certain critical frequency, equalizingthe attenuation and equalizing Vthe phase shift in these channels tooffset de- `.partures occasioned by said separation, and thenrecombining "the ycomponents to produce a resultant signal like 4'theinitiall signal. y

2. The method ofcompensatingfor the ldeparture from, exact cut-off`given by filters when'separatingomponents of a' signal wave' 1nseparate channels according to frequency,-

whichconsists in equalizing the attenuation andpha'se'shift in theseparate channels and thereafter combining the currents in thosechannels. .A

` 3. In combination, a channel, .branches therefrom, means to effectanapproximate separation of the components of a signal current wave fromsaid channel into said branches according to frequency, means in e saidbranches to compensate for attenuation andphase shift, and kmeans, tocombine the components in theA branches whereby the resul'tantfcurrentwill have` the same signal y form as the initial current. 14. The methodof separa-ting components of a signal currentaccording to frequency andrecombining them, which consists in iil- ,tering such components apartwithso much distortion at frequencies near theseparation 'frequency asmay be unavoidable, and ythen ycompensating the separated currentcompovnents forattenuation and phase shift so that when recombined theresultant current will correspond to the initial current.

. 5.. In thel method of combining currents of adjacent frequency rangeswith slight overlap, the step which consists in compensating themforattenuation and phase shift at frequencies which overlap, so that whenthey are combinedthere lwill be no irregularity of amplitude nor ofphase shift at those frequencies.

6. Incombination, a line'good for transmitting currentsv of frequenciesonly from a tod, means at the sending end to enerate signal currentswith components at requencies from 0 tol bv, means to separate theseinto two channels with ranges from 0 to a and t y lfrom a to b, means togenerate a carrier at The combined currents at the junction of c andmodulate it by the said low'er range, thus producing a modulated outputof range within ca to c-l-a, means to apply such output and the currentof rangen to b to the'said line, means toseparate these ranges at thereceiving end, a detector to getv the current components from O to afroml the higher'received range, compensators for attenuation and forphase shift for the sepa-` `channels below and above a, stepping thelower'such range up to a range lying between b and d, then transmittingboth ranges on said line, and, ,at the: receiving end, separating themand stepping the higher range downto 0 to a, compensating theseseparated ranges for attenuation and phase shift and combining them'toproduce a sicnal wave like the initial wave at the transmitting end,

the frequency. values having magnitudes in the order 0, a, b, anddwithd-"b 2a.'

8. -The method of effecting transmission of signal currents-of componentfrequencies from zero up to a considerablevalue over a line which Willnotv transmit' directly currentsv of low frequency, which consists in yseparating off thelow frequency components,

stepping them up in frequency above the upper limit of thesignal'frequency frange, transmitting both ranges, and, at the receivingend,'stepping the higherrrange down to the corresponding initial values,compensat-` ing in bothran'ges for attenuation and phase andthenrecombining the components to produce a resultant'signal like theinitial signal.`

In testimony whereof, we have signed our names to this specificationthis 9th day of June, 1927. y

HARRY NYQUIST. KENNETH W. PFLEGER.

shift and combining and applying therethe said last-mentioned range andthe said second range, and at the receiving end, separating the secondrange and third range currents, stepping the third range currents downto their Yinitial values as in the-first range, andthen combining theresultant currents to get a signal currentcorresponding to the initialsignal current at the sending end.

l0. In combination, means to produce a signal current having frequencycomponents lying in two ranges which may be designated first and second,a line adapted to transmit frequency components in the second rangeandanother range which may be designated .as the third, means at thesending end to separate the components of the first and-second rangesand to shift the frequency of the first range components into vthe thirdrange, means to apply the currents of the second and third ranges to theline, and. at the receiving end, means to separate these ranges, meansto step the currents in the third range back to the initial' values ofthe first range, means to compensate the first range currents and thesecond range `currents for attenuation and phase shift, and means tocombine and applythe currents so compensated to produce a resultantsignal wave like the initial signal Wave at the sending end.

llt; The method of transmitting a signal, which consists in separatingits components into respective channels approximately above and below acertain critical frequency, equalizing the attenuation and equalizingthe phase shift in each of vthese channels to Oifsct departuresoccasioned by said separation,

